1. Technical Field
The present invention relates to an optical device, a detection apparatus, an electronic apparatus, and a method for producing an optical device.
2. Related Art
Recently, the demand for a sensor chip to be used for medical diagnoses, tests for foods and beverages, etc. has been increasing, and the development of a highly sensitive and small sensor chip has been demanded. In order to respond to such a demand, various types of sensor chips such as electrochemical sensor chips have been studied. Among these, for the reasons that integration is possible, the cost is low, measurement can be performed in any environment, etc., sensor chips using a spectroscopic analysis utilizing surface plasmon resonance (SPR), particularly, surface-enhanced Raman scattering (SERS) have drawn increasing attention.
Here, the term “surface plasmon” refers to an oscillation mode of an electron wave that is coupled to light depending on boundary conditions specific to a surface. As a method for exciting surface plasmons, there is a method in which a diffraction grating is imprinted on a metal surface to couple light to plasmons or a method in which an evanescent wave is used. For example, as a sensor utilizing SPR, a sensor configured to include a total reflection prism and a metal layer which comes into contact with a target substance formed on the surface of the prism is known. According to such a configuration, whether or not a target substance is adsorbed, for example, whether or not an antigen is adsorbed in an antigen-antibody reaction, or the like is detected.
However, while propagating surface plasmons exist on a metal surface, localized surface plasmons exist on a metal fine particle. It is known that when the localized surface plasmons, i.e., the surface plasmons localized on the metal microstructure on the surface are excited, a significantly enhanced electric field is generated.
It is also known that when an enhanced electric field formed by localized surface plasmon resonance (LSPR) using metal nanoparticles is irradiated with a Raman scattered light, the Raman scattered light is enhanced by surface-enhanced Raman scattering phenomenon, and therefore, a sensor (detection apparatus) with high sensitivity has been proposed. By using this principle, it becomes possible to detect a small amount of various substances.
An enhanced electric field is large around metal particles, particularly in a gap between adjacent metal particles, and therefore, it is necessary to retain a target molecule in a fluid sample in the gap between metal particles. For example, in Patent Literature 1 (JP-A-2009-222401) or Non-Patent Literature 1 (P. Freunscht et al., “Surface-enhanced Raman spectroscopy of trans-stilbene adsorbed on platinum or self-assembled monolayer-modified silver film over nanosphere surfaces”, Chemical Physics Letters, 281 (1997), 372-378), as schematically shown in FIGS. 1A and 1B, on a metal surface of a sensor substrate 200, a self-assembled monolayer (SAM) film 201 is formed, and a target molecule 202 or 203 is adsorbed thereon, whereby the detection sensitivity of SERS is improved. Further, in Patent Literature 2 (JP-A-2008-177283), by repeating a formation step, a self-assembled monolayer film having few defects is formed.
In Non-Patent Literature 2 (Olga Lyandres et al., “Real-Time Glucose Sensing by Surface-Enhanced Raman Spectroscopy in Bovine Plasma Facilitated by a Mixed Decanethiol/Mercaptohexanol Partition Layer”, Anal. Chem., 77 (2005), 6134-6139), as schematically shown in FIG. 2, by forming and mixing two types of SAMs 210 and 211 having an organic group with a different length, a capture space 212 having a hydrophilic group and a hydrophobic group is formed, and a target molecule (glucose) 213 is adsorbed in the capture space 212, whereby the detection sensitivity of SERS is improved.
When one type of SAM 201 is formed, in the case of a large target molecule 202 such as a protein shown in FIG. 1A, there are many adsorption sites between the target molecule 202 and the surface of the SAM 201, and the target molecule 202 is held by multipoint adsorption. On the other hand, in the case where a target molecule 203 shown in FIG. 1B has a low molecular weight (for example, a volatile organic compound (VOC) such as toluene, xylene, acetone, or isoprene), since the molecule is small, the number of adsorption sites is about 1, and therefore, since the adsorption force is small, sufficient detection sensitivity cannot be obtained.
On the other hand, as the method for forming two types of SAMs 210 and 211 shown in FIG. 2, there are the following two methods: one is a method in which a substrate is immersed in a solution obtained by mixing different SAM constituent molecules at a given ratio, whereby two types of SAMs are formed; and the other is a method in which one SAM is formed in advance for a relatively short formation period, and thereafter, the other SAM is formed.
However, the above-described methods have a problem, for example, the same type of SAM molecules aggregate, or an SAM which is easily formed is preferentially formed, and therefore, it is difficult to form the capture space 212 in a regularly arranged pattern as shown in FIG. 2. Further, the size (the width in the arrangement direction) of the capture space 212 as shown in FIG. 2 cannot be controlled.